1. Field of the Invention
The invention relates to a method for manufacturing an optical fiber cable element having a plurality of optical fibers which are arranged with excess length inside a pressure-tight metal tube. Means for protecting the optical fibers and for longitudinal sealing are provided inside the metal tube.
2. Description of the Prior Art
Optical fiber cables having a plurality of optical fibers which are arranged in a pressure-tight metal tube are preferably used as submarine cables. The aim is to manufacture great lengths in one piece since the splicing of submarine cables is unreliable and costly. Since it is not possible to insert the optical fibers into prefabricated tubes of great length, the only remaining possibility is the continuous sheathing of optical fibers with longitudinally seam-welded tubes, i.e., the insertion of the optical fibers into a slotted tube which is still open.
Such a method is described in U.S. Pat. No. 4,759,487. A metal band, preferably of stainless steel, drawn off a supply reel, is gradually shaped into the slotted tube in a plurality of shaping steps. The optical fibers are run into the still-open slotted tube and the longitudinal seam of the slotted tube is welded together. Laser welding is used as the welding method. After the welding, the diameter of the metal tube is reduced, the drawing force being produced by a drawing drum. The excess length of the optical fibers is produced by having the optical fibers introduced into the tube by a separate tube which runs inside the welded and drawn tube, the end of which extends up to the area of the drawing drum. A gas is conducted through the tube and "lubricates" the optical fibers as they pass through the tube and ensures that the optical fibers rest against the outer circular arc of the welded tube. A further tube extends into the area of the drawing drum in the interior of the welded tube and is provided for longitudinally sealing the optical fiber cable. This second tube is longer than the tube carrying the optical fibers and transports a jelly-like compound into the welded tube which fills up the free space between the optical fibers and the inside wall of the welded tube.
When the production rate is approximately 20 m/min, more than three days are required to produce a length of 100 km of cable element. The aim is to fabricate the length to be produced without stopping, since each production stop is associated with an increased fault rate.
The known method has a number of disadvantages. For example, the handling of the optical fibers, i.e., the winding-off, the guiding through the tube and the blowing in of air, can lead to a fracture of the glass fiber even though it is coated with a plastic layer. Furthermore, the excess length to be achieved in this method is insufficient for many applications. The excess length depends on the outside diameter and the wall thickness of the welded tube. It is also not possible to check the longitudinal sealing during production. Thus, in this method, faults or defects can only be found in the completed cable, i.e., after more than three days of production.